An electric utility grid generally has many independent energy sources energizing the grid and providing power to the loads on the grid. This distributed power generation has become increasingly common throughout the world as alternative energy sources are being used for the generation of electric power. In the United States, the deregulation of electric companies has spurred the growth of independent and alternative energy sources co-existing with the electric utility. Rather than have dedicated energy sources for a particular load, these alternative energy sources can tie into the grid and are used to supplement the capacity of the electric utility.
This growth of alternative energy resources has offered large opportunities for innovation and market development in the hardware and infrastructure components that support these alternative resources. Many alternative resources connect to the grid through power electronic inverters and converters. Inverters or other types of power converters are configured to convert direct current (DC) power taken from many types of alternative energy sources into alternative current (AC) power suitable for connection to the electric utility grid. Other examples include direct DC power from fuel cells or solar cells, or rectified power from micro-turbines or small wind power devices.
Two basic architectures for interconnection to the electric grid, corresponding to AC and DC sources are possible. In a “DC-side” system, the alternative energy resources are interconnected at their terminals to form a single large DC source. A single large inverter performs the power conditioning and grid interconnection process of the single DC source. In an “AC-side” system, each resource has its own power converter, and interconnection to the electric grid takes place either within the electric grid or at a conventional AC transformer that interfaces to the grid. Combinations of these two systems are possible, in which DC-side connections build the source power up to a desired level, while multiple AC-side connections act together to form a large effective grid source. Most present photovoltaic installations use DC-side connections because of control complexities and high costs associated with inverters. Low-cost modular power converters are required for implementation of AC-side connections.
At least three major challenges arise in trying to develop power converters suitable for grid connection. A first challenge is basic cost. Conventional power converter designs impose costly control and filtering requirements. There is much promise in unconventional designs, which scale to a wide range of power levels. A second challenge is expandability. A parallel interconnection of multiple units as power sources can give rise to control issues. As such, individual power converters need to support this form of interconnection without creating control problems. A third challenge is to meet requirements of codes and standards for alternative energy resource connection to the grid, which are documented in the Institute of Electrical and Electronic Engineers, Inc (IEEE) Standard 1547™ and Underwriters Laboratory (UL) Standard 1741, which are directed to standards for interconnecting distributed resources with electric power systems. This third challenge is a topic of current global research. Many known approaches tend to add considerable complexity and cost to the power converter. A major requirement for grid-connected power converters imposed by IEEE Standards 1547 and UL Standard 1741 is “anti-islanding” protection. When the grid connection is lost or otherwise stops functioning, the photovoltaic power converter is required to shut off to avoid “back-feed” into the corresponding local distribution network. This is an important safety issue so repair personnel and others can be assured that there will not be unexpected sources of power on lines thought to be de-energized. There are also important operational considerations such as voltage and phase synchronization when attempting to re-connect an islanded portion of the utility grid.
Generally, techniques or methods used to detect islanding conditions at the individual power converter fall in one of two categories, passive and active. Passive methods typically monitor waveform parameters and disable output power if these waveform parameters sufficiently deviate from specified conditions. In this configuration, the method is strictly an observer. Active methods for detecting islanding conditions typically introduce deliberate changes or disturbances into the grid, then monitor the system response. Thus, active techniques are generally described as “perturb and observe” control processes.
Typically, phase-locked loops or similar methods are used to synchronize the output power of the power interface with the grid. As such, the current injected into the grid is made to the desired grid operation, not the actual grid operation. The issues associated with synchronization are well understood and require additional control processes for startup and transient operations.
Therefore, a need exists for a power converter that provides anti-islanding protection and overcomes the problems noted above and others previously experienced for addressing issues of cost, expandability, regulation and connectability. These and other needs will become apparent to those of skill in the art after reading the present specification.